1. Field of the Invention
The present invention relates in general to a radio-frequency tag communication device configured to effect radio communication with radio-frequency tags for writing and reading information on and from the radio-frequency tags, and more particularly to techniques for suppressing a received leakage signal which is a part of a transmitted signal.
2. Description of the Related Art
There is known an RFID (Radio-Frequency Identification) communication system wherein a radio-frequency tag communication device (interrogator) reads out information, in a non-contact fashion, from small-sized radio-frequency tags (transponders) on which desired information is written. In this RFID communication system, the radio-frequency tag communication device is capable of reading out the information from the radio-frequency tags, even where the radio-frequency tags are contaminated or located at positions invisible from the radio-frequency tag communication device. For this reason, the RFID communication system is expected to be used in various fields, such as management and inspection of articles of commodity.
The radio-frequency tag communication device indicated above is usually arranged to transmit a predetermined transmitted signal from an antenna device toward the radio-frequency tags, and to receive through the antenna device a reply signal transmitted from each radio-frequency tag which has received the transmitted signal. Thus, the radio communication is effected between the radio-frequency tag communication device and the radio-frequency tags. Where a receiver antenna device is positioned so as to be able to receive the transmitted signal, a part of the transmitted signal may be received by the receiver antenna device, as a leakage signal of a high strength mixed in a received signal received by the receiver antenna device, giving rise to a problem of a reduced signal/noise ratio of the received signal. In view of this problem, there have been proposed techniques for eliminating the leakage signal received by the receiver antenna device from a transmitter antenna device. Patent Document 1 discloses a direct conversion receiver, as an example of such techniques.    Patent Document 1: JP-2001-102942A
The radio-frequency tag communication device is preferably provided at its receiver portion with a homodyne wave detector circuit as a simple receiver circuit. In the prior art techniques indicated above, it is not possible to sufficiently prevent mixing of a direct component in the detected wave, where the leakage signal transmitted from a transmitter portion is included in the received signal. Accordingly, the signal/noise ratio is lowered, leading to a drawback of a reduced distance of communication. For this reason, there has been a need of developing a radio-frequency tag communication device having a receiver portion which is simple in construction and which is capable of sufficiently eliminating the leakage signal received from a transmitter portion.